The present invention relates generally to fiber optic communications systems and methods, and more particularly, to the use of bandwidth efficient modulation techniques to transmit information over a wavelength division multiplexed (WDM) optical link.
Previous solutions for transmitting information over a wavelength division multiplexed link involved increasing the speed of simple on-off-keying until the capacity of a given WDM carrier passband is filled. Unfortunately, simple on-off-keying is limited to 1 bit/Hz at best. Moreover, increasing the speed of on-off-keying requires increasingly expensive and noise sensitive electronics.
Numerous patents have been issued relating to transmission of data via optical communication systems that were uncovered in a search relating to the present invention. These patents disclose apparatus and methods relating to optical modulators, intermodulation products, amplifiers, receivers, heterodyne communication frequency multiplexed carriers, linearizers, signal sources, frequency division multiplexing, intensity modulation, mixers, spread spectrum CDMA systems, harmonic interference, optical carrier filtering, and polarization schemes. However, these patents do not address systems or methods that intensity modulate an optical carrier with one or more microwave carriers that are higher order modulated.
Therefore, it would be advantageous to have systems and methods that improve data transmission over a wavelength division multiplexed optical link.
The present invention increases the data rate capacity per optical carrier on a wavelength division multiplexed (WDM) system with carrier spacing of 100 GHz or less. The present invention intensity modulates an optical carrier with one or more microwave carriers that have been higher order modulated (M-ary ASK, PSK, QAM, etc.). As a result, more than 1 bit/Hz of optical channel can be transmitted.
More specifically, the present invention provides for systems and methods that improve transmission of information over a wavelength division multiplexed optical link. One or more bandwidth efficient modulation (BEM) modulators for modulating data for transmission are used. The BEM modulators use microwave carrier signals at a predetermined first carrier frequency. The modulators incorporate filtering so that only the main spectral lobe remains, and may also incorporate pre-equalization to compensate for non-ideal link aspects. Among these aspects are fiber dispersion and the higher order terms in the optical spectrum generated by the intensity modulation of the optical carrier. The optical carrier is intensity, not amplitude modulated.
Frequency converters are then used to shift the lobes to frequencies such that they may be combined into a composite microwave spectrum that most efficiently uses the WDM channel passband. This composite microwave signal is then used to intensity modulate the WDM optical carrier. This WDM optical carrier is then transmitted on a fiber and received using normal WDM optical apparatus. The received composite microwave spectrum of each WDM channel is then processed as appropriate, in that the lobes are separated with filters, shifted to their original frequencies and demodulated to generate the originally transmitted data.